Consequences of an ecosystem state shift for nitrogen cycling in a desert stream

Ribot, Miquel; Grimm, N. B.; Pollard, Lindsey D.; Von Schiller, D.; Handler, Amalia M.; Martí, Eugènia. Limnology and Oceanography : doi: 10.1002/lno.12076 (2022)   DIGITAL CSIC  

Cessation of cattle grazing has resulted in the reestablishment of wetlands in some streams of theU.S. Southwest. Decades of cattle grazing prevented vascular plant growth in Sycamore Creek (Arizona, U.S.A.),resulting in stream reaches dominated by diatoms andfilamentous green algae. Establishment of vascular plantscan profoundly modify ecosystem processes; yet, the effects on nitrogen (N) cycling remain unexplored. Weexamined the consequences of this ecosystem state shift on N cycling in this N-limited desert stream. Wecompared results from whole-reach ammonium-N stable isotope (15NH4+) tracer additions conducted before(pre-wetland state) and 13 yr after (wetland state) free-range cattle removal from the watershed. Water columnestimations showed that in-stream N uptake and storage were higher in the pre-wetland than in the wetlandstate. N turnover was also higher in the pre-wetland state, indicating that assimilated N was retained for shortertime in stream biomass. In addition, N uptake was mostly driven by assimilatory uptake regardless of the ecosys-tem state considered. Water column trends were mechanistically explained by the fact that the dominant pri-mary uptake compartments in the pre-wetland state (i.e., algae and diatoms) had higher assimilatory uptakeand turnover rates than those in the wetland state (i.e., vascular plants). Overall, results show that the shift inthe composition and dominance of primary producers induced by the cessation of cattle grazing within thestream-riparian corridor changes in-stream N processing from a dominance of intense and fast N recycling to aprevalence of N retention in biomass of primary producers.